Nonlinear optical crystal such as lithium niobate or lithium tantalate single crystal has a high secondary nonlinear optical constant. When a periodic domain inversion structure is formed in the above crystals, a second-harmonic-generation (SHG) device of a quasi-phase-matched (QPM) system can be realized. Further, when a waveguide is formed within this periodic domain inversion structure, the high-efficiency SHG device can be realized and further, applied to optical communication, medical science, photochemistry, various optical measurements over a wide range.
In a harmonics generating device disclosed in United States Patent No. 2007-0189689A, the periodic domain inversion structure is formed within a channel type optical waveguide, whereby a fundamental wave incident on the optical waveguide is subjected to wavelength conversion into higher harmonics. In a harmonics generating device disclosed in WO 2006/41172 A1, a thin plate of a ferroelectric single crystal is adhered to a supporting substrate and thereon, an upper-side substrate is adhered through a buffer layer and an adhesive layer, thereby forming the channel type optical waveguide within the thin plate. Further, the periodic domain inversion structure is formed within the optical waveguide, whereby a fundamental wave incident into the optical waveguide is subjected to wavelength conversion into higher harmonics.